Double-Leg Kick (Dolphin Kick) Basics Part I: Kicking Depth

After watching NCAA footage, one fact becomes increasingly clear each year: double-leg kicking (dolphin kick) correlates with success [arguably in all strokes, including breast...].

This trend is clear, but not too long ago, double-leg kicking was discouraged, as "experts" thought surface swimming was superior to underwater double-leg kicking. As you see, we have become far since this era, as more research surmounted and people went against traditional thinking.

This series will discuss the basics of double leg kicking and before we delve into the biomechanics and current unknowns, understanding why double leg kicking can be faster than surface swimming is mandatory.

First things first, the further you are underwater, the lower the drag coefficient. Marinho et al. (2009) measured the amount of drag during gliding underwater. 

The deeper the better is a common motto and although deeper water reduces drag coefficients, there is a point of diminishing returns. Table-1 notes a decreasing drag coefficient with decreasing depth. However, figure-1 makes the point of diminishing return clear, as 2.0 - 2.5 meters of depth is the where the drag coefficient levels off.

Figure-1

Table-1 Marinho 2009

Conclusion
As you see, the deeper the better, but there is a point of diminishing returns. Moreover, each individual has a different profile in the water and different drag coefficient. Combine this with general biomechanics and you have a much more complicated equation. Once again, research only takes us this far, then the principle of individuality applies.

Reference
  1. Marinho DA, Reis VM, Alves FB, Vilas-Boas JP, Machado L, Silva AJ, Rouboa AI. Hydrodynamic drag during gliding in swimming.J Appl Biomech. 2009 Aug;25(3):253-7.
By Dr. G. John Mullen received his Doctorate in Physical Therapy from the University of Southern California and a Bachelor of Science of Health from Purdue University. He is the founder of the Center of Optimal Restoration, head strength coach at Santa Clara Swim Club, creator of the Swimmer's Shoulder System, and chief editor of the Swimming Science Research Review.

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